1. Field of the Invention
The present invention relates to an arrangement for comparing two temporally separated bursts of signal at two different frequencies in particular an interrogation pulse and a reply pulse in a DME (Distance Measuring Equipment) system for aid to navigation.
2. Description of the Prior Art
DME systems, both of the DME/N type (used in the ILS (Instrument Landing System)) and of DME/P type (used in the MLS (Microwave Landing System) navaid systems), provide the aircraft with distance information by measuring total round-trip time between a transmission of an interrogation signal from an airborne transmitter and a receipt of a reply signal from a ground transponder.
From a strictly theoretical point of view, and therefore by not taking into account air traffic and morphology of the particular ground are where the DME system really operates, it is sufficient that the transmitter only sends a so-called interrogation pulse and the transponder, after receiving the interrogation pulse, transmits in turn only a so-called reply pulse.
Since the transponder takes a certain time to reply, it has been established by standardization authorities in the field of air navigation that the reply pulse is transmitted by the transponder with a fixed and pre-established delay with respect to the instant in which the interrogation pulse is received. The airborne equipment, therefore, takes into account such fixed and pre-established delay for the correct evaluation of distance.
Hence, during manufacturing and installation, there is the problem of being able to calibrate the transponder so that it provides such pre-established delay with precision.
Both of the above-mentioned pulses (i.e., the interrogation and reply pulses) are transmitted by modulating the amplitude of two carriers at two different frequencies which are both about 1 GHz, and are spaced apart exactly 63 MHz from each other; the pulses have the same form and the same duration (i.e., about 3 microseconds); the fixed and pre-established delay is about 50 microseconds; the precision with which such delay must be observed by the transponder is a few nanoseconds.
An obvious solution consists in providing at the input of an oscilloscope both pulses in such a way as to display them and then proceed with the measurement; but an oscilloscope having the characteristics necessary for allowing such measurement with the parameters set forth above is extremely expensive.
As a result in the prior art, it is common practice to place before the oscilloscope, a simple envelope detector, in such a way that the band necessary for the oscilloscope is a few MHz. For the envelope detector, a known diode and capacitor configuration is used in such a way that distortion introduced by said detector is limited as much as possible.
However, such a solution introduces comparatively large errors, with respect to the strict requirements, due to diode offset and above all due to the fact that both the measuring system and the envelope detector operate at two different frequencies and therefore they introduce different delays for the two (interrogation and reply) pulses.